1. Field of the Invention
This invention relates generally to a pneumatic booster for assisting in operating a master cylinder of brake and clutch systems of a vehicle and particularly to a pneumatic booster of the pull type in which input and output forces of the booster are a pulling force.
2. Prior Art
Generally, the prior art boosters of the type described comprise a hollow body and a movable wall movably received in the hollow body to divide it into a low pressure chamber and a high pressure chamber, the low pressure chamber being kept at a predetermined low pressure while the high pressure chamber is selectively caused by valve means to communicate with either the low pressure chamber or an external pressure source. In the case of the booster of the negative pressure-type in which the low pressure chamber communicates with a negative pressure or vacuum source such as an intake manifold of a vehicle engine, the ambient atmosphere is used as the external pressure source of the high pressure chamber. On the other hand, in the case of the booster of the type in which the low pressure chamber is in communication with the ambient atmosphere, an air compressor is used as the external pressure source. In both cases, a boosting force is produced by a pressure differential between the low and high pressure chambers.
The movable wall has a rear portion slidably extending through a rear wall of the body, and an internal bore extending through the rear portion. A plunger is mounted in the internal bore and connected to an input member for axial movement therewith along the internal bore. The valve means has a valve seat formed on the plunger on which valve seat a valve element is adapted to rest to block off the communication of the high pressure chamber with the external pressure source. The input member is coupled to the plunger through a ball joint arrangement so that the valve element can be positively seated on the valve seat even if the input member is subjected to lateral movement. More specifically, the input member in the form of a rod has a rounded end, and the plunger has a ball-receiving socket in which the rounded end of the input member is received and retained to form the ball joint. An operating wire is secured to the input member and is connected to a brake pedal or a clutch pedal, and upon operation of such a pedal, the wire is pulled to move the input member. The prior art boosters of this type are disclosed in Japanese Patent Application Laid Open (Kokai) Nos. 58-89451 and 58-97550.
By virtue of the provision of the ball joint, the seating of the valve element on the valve seat is not adversely affected by the lateral movement of the input member. However, when the pedal is abruptly operated, for example, to apply an abrupt braking action, the seating of the valve element on the valve seat tends to be unstable, thereby causing a pressure leakage of the valve. This difficulty is considered to arise from the fact that the plunger is not accurately guided along its longitudinal axis.
Another conventional booster as disclosed in Japanese Patent Publication No. 57-36183 comprises a plunger or rod slidably extending through a cover member of a housing or body and a movable member slidably received in the housing, so that the movement of the plunger along its axis is guided by the cover member and the movable member. However, since the movable member is movable relative to both the plunger and the housing, this conventional booster suffers from the problems encountered with the above-mentioned conventional booster.